[llvm] [DAGCombiner] Add combine for vector interleave of splats (PR #151110)

[Paul Walker via llvm-commits]

================
@@ -25465,6 +25502,24 @@ SDValue DAGCombiner::visitCONCAT_VECTORS(SDNode *N) {
   return SDValue();
 }
 
+SDValue DAGCombiner::visitVECTOR_INTERLEAVE(SDNode *N) {
+  // Check to see if all operands are identical.
+  if (!llvm::all_equal(N->op_values()))
+    return SDValue();
+
+  // Check to see if the identical operand is a splat.
+  SDValue Op0 = N->getOperand(0);
+  SDValue Splat = DAG.getSplatValue(Op0);
+  if (!Splat)
+    return SDValue();
+
+  // Simply replace all results with the first operand.
+  SmallVector<SDValue, 4> Ops;
+  Ops.append(N->op_values().begin(), N->op_values().end());
+  SDValue Res = CombineTo(N, &Ops);
+  return Res;
----------------
paulwalker-arm wrote:

```suggestion
  return CombineTo(N, &Ops);
```

https://github.com/llvm/llvm-project/pull/151110